The invention relates to a predistortion linearizer for a power amplifier using a bridge topology.
Telecommunications payloads for spacecraft must be able to transmit large volumes of information with the lowest device mass possible. Because novel, efficient modulation methods and multi-carrier methods require increasingly large bandwidths, broadband linearizers are important components of transmission units that are transparent for carrier frequencies and a wide variety of modulation methods.
Power amplifiers exhibit nonlinear behavior, which can vary within a production lot and is highly dependent on the frequency. Linearizer bridges according to the present state of the art only allow the nonlinear compensation thereof to be adapted for a portion of the operating frequency bandwidth, but the linearization does not correspond to the required curve for the remaining frequencies within the useful band.
A linearizer bridge is based on the vectorial addition of a linear, which is to say power-independent, signal and a non-linear signal, which is to say a signal subject to distortion. The vectorial division and addition of the signals of the two branches takes place, for example, in 90° hybrids at the input or output of the circuit.
In the nonlinear branch, the signal first passes through the distortion generator. As the signal power increases, the signal deviates from the small-signal behavior at the distortion generator output with respect to amplitude and phase. In most cases, both the amplitude and phase undergo compression. Using a variable phase shifter, for example in the linear branch, or in each of the two branches, the necessary relative phase shift between the linear and nonlinear branches can be adjusted at small-signal levels.
In the nonlinear branch, or in both branches, a variable attenuator makes it possible to adjust relative attenuation between the nonlinear and linear branches.
At frequencies in the double-digit GHz range, the nonlinear behavior of amplifiers is highly dependent on the frequency. Parasitic effects within the linearizer bridge make it more difficult to control the frequency response of the linearizer. In general, the desired properties can only be adjusted in a limited frequency range.
DE 195 28 844 C1 describes a “method and a system for linearizing the gain characteristics and phase-frequency characteristics of traveling-wave tubes and transistor amplifiers at different power levels”. To this end, the signal is divided into two branches and again united into an output signal, wherein the signal is divided between two nonlinear partial networks having compressing gain characteristics, and gain compression or gain expansion of the partial signals is generated at two gates by reflection behavior, which differs depending on the signal level. The nonlinear, gain-expanded signal and the gain-compressed signal are united into the output signal using an adding network.
A “broadcast transmission system with distributed correction” (DE 600 06 102 T2) is known for compensating for distortions in a digital transmission system. To this end, a transmission system is provided with a signal path, wherein a first component of a plurality of components, which are located in the signal path, subject the information signal to nonlinear distortion shifts, and a further plurality of components, which are third and fourth components, are used to modify the information signal so as to compensate for distortions in inverse order to the first plurality of components. The first component comprises an amplifier, the second component comprises a predistortion filter, the third component comprises a nonlinear corrector, and the fourth component comprises a linear equalizer. The fifth component comprises a filter and the sixth a second linear equalizer.